Enabled depth buffer creates white ghost object

Hi there,

I'm "learning" opengl for some days now and try to solve all the tutorials I can find on the web.
The first "hello triangle" took quite some time, but I already got my hands on some colored cube chapters by now.

And thats where I get my first unsolvable problem (at least by me). That's why I'm here, and I hope you can help me.

--- The problem ---
When I enable the depth buffer, my code generates a strange "ghost" object after some seconds. When I disable the depth buffer, running the code won't generate it, but my cube looks wierd (because of the missing depth test).
I'm using glm for mathematics, GLEW 1.9 and GLFW 3.0.1.
For demonstration purposes, watch this short clip on youtube, to see my problem in action:

EDIT: Looks like I can't post links or embedded-videos, so you have to do it manually if you want to watch it.
Just add the following "watch?v="-ID to the youtube-URL: TYKMR7lkp30 (Error shows up at 0:11)

--- The code ---
I know my code is not the best.
I have been coding .NET (C#) for years now and learn opengl and c++ now the "learning by doing" way for a week
I try to shorten it to the important parts:

2) Creating the buffers and that stuff:
(The first two lines create a custom made "cube" object. The constructor takes X/Y/Z-coords in world space and the length of the cubes sides. The second line creates the vertices for the cube.)

Unfortunately, I found the "bug" already - it is not within the code, it is my computer.
Tested the compiled exe on two other systems (one intel + nvidia, like my programming rig, and one amd+ati), both did not produce the glitch.

I checked my running processes. Do you know "Trillian", some IM app for multiple protocols? It has an "ingame-chat"-function. It detects running games and allows you to use a keybinding to open up a chat window ingame without ALT+TABing. So, when this feature detects the game (thats why it happens some seconds after it was run and not instantly), it creates this bug. Simply deactivated the ingame-chat-function in Trillian and tadaaa - problem solved.